Project Summary Multicellular organisms encode small non-coding RNAs called microRNAs (miRNAs) to regulate gene expression by exerting repression on their target mRNAs. Defective synthesis of miRNAs and abnormal miRNA activity are associated with developmental defects and diseases, including human. Thus, miRNAs are being targeted or used for both diagnostic and therapeutic purposes. However, there are several critical caps in miRNA biogenesis. Our long-term goal is to elucidate how miRNA levels are precisely controlled. miRNAs are derived from the primary miRNA transcripts (pri-miRNAs), which contain one or more imperfect stem-loop (s) where miRNAs reside. Pri-miRNAs are co-transcriptionally processed. However, it is not clear how the processing complex is formed and recruited to pri-miRNAs. Moreover, most pri-miRNAs are non-coding RNAs, and therefore, are likely targets of the nuclear RNA quality control machinery (QC). Indeed, failure to protect pri-miRNA from degradation causes development defects. However, the process protecting pri-miRNA from degradation is not defined. The MOS4-assoicated complex (MAC) from plants is a conserved complex composed of core and accessory components. The ortholog complexes of MAC in metazoan and fungi are known as the PRP19 complex and the nineteen complex, respectively. They associate with spliceosome and are required for splicing, DNA damage repair and genome stability. Our previous study leads us to propose that MAC links pri- miRNA transcription, processing and stability together. We hypothesize is that MAC is important for co- transcriptional processing and stability of pri-miRNAs. Our specific aims will test the following hypotheses: 1) MAC functions as a whole complex to modulate miRNA biogenesis; 2) MAC binds pri-miRNAs to protect them from XRN2/3 activity; 3) Phosphorylation-dependent ubiquitin ligase activity of MAC3 is critical for miRNA biogenesis. Collectively, our research will produce a broad impact by characterizing the roles of MAC in the transcription of pri-miRNAs, formation of processing complex, stabilization of pri-miRNAs and regulation of the activity of processing complex. Consequently, our research will improve our ability to understand the miRNA- mediated gene regulation and to develop miRNA-related technologies that can be used to improve human health and agricultural trait of crops